The invention relates generally to audio entertainment systems for use in automobiles, trucks and other motor vehicles. More particularly, the invention relates to certain loudspeaker systems to be employed in such vehicles such that the so-called stereo imaging created by such audio entertainment systems may be greatly enhanced.
During the past several years various efforts have been made to improve the overall sound quality of the audio entertainment systems which are installed in automobiles and other motor vehicles. While improvements in the electronic components of such systems, such as amplifiers, radio tuners and tape playback decks, have been significant, those improvements have occurred concurrently with and largely as the result of corresponding improvements in audio systems designed for home use. Developments in the design of loudspeaker systems have also paralleled those in the home audio field, but they have done so to a somewhat more limited extent due to problems faced in the automotive environment which either are not present in the home environment or are of significantly less importance there.
In a stereo or other multi-channel audio system, the listener perceives the desired stereo effect produced by such a system only when he hears the correct proportion of sound emanating from loudspeakers responding to the signals produced by two discreet channels, each bearing a particular combination of sounds, i.e., when there is proper channel separation. Thus, the concept of sound directionality is of critical importance in producing this stereo effect, commonly referred to as the stereo image.
The stereo image is adversely affected when the sound waves reach the listener's ear as the result of reflection rather than direct radiation from the loudspeaker itself, for the reflected sound not only does not reach the ear at the same time as does the directly radiated sound but is perceived to have originated at the reflective surface instead of the loudspeaker. Since the optimum stereo image results from carefully chosen loudspeaker location, it follows that loudspeaker placement which creates sound which is perceived to have originated at points other than the positions of the loudspeakers would deleteriously affect the stereo image. Thus, a crucial function of loudspeaker placement is to maximize the ratio of direct to reflected sound reaching the listener's ear in a listening environment.
The placement of the transducers which produce the middle and higher sound frequencies is of particular importance to the creation of an optimum stereo image, for it is well known that the middle and higher sound frequencies are, relative to the lower frequencies, highly directional due to their shorter wavelengths. Evidence of this fact is the observation that, as the frequency of the sound increases, the radiation pattern of the sound waves becomes less hemispherical and more like a narrow, directional beam. Lower-frequency sounds, on the other hand, are essentially nondirectional due to their longer wavelengths, and they therefore make a negligible contribution to stereo imaging. For example, it has been demonstrated that, at equal loudness levels, sounds below 125 Hertz which are reflected by an order of one to two are perceived by the listener as being directly radiated to that listener. Accordingly, it is the middle and higher frequencies which are, as a result of their directional character, almost exclusively responsible for determining the extent to which a proper stereo image is produced by the loudspeakers.
While the placement of the middle- and higher-frequency transducers relative to the ear of the listener therefore determines the ratio of direct to reflected mid- to high-frequency sound and thus affects the extent to which proper clarity and stereo imaging is achieved in any environment, e.g., a living room or a concert hall, such loudspeaker placement is of critical importance in achieving proper stereo imaging in an automobile. This importance results from the fact that an automobile passenger compartment is a small-volume listening environment, highly crowded with sound absorbing surfaces, sound-reflecting surfaces and obstructions which present physical barriers to the transmission of sound waves directly from possible loudspeaker locations to the ears of the listener. Thus, the passenger compartment is an inherently hostile environment for the creation of a proper stereo image by the highly directional middle and higher frequency sounds absent proper placement of the middle- and higher frequency transducers.
Moreover, because of the small volume of the automobile interior and the presence of numerous sound-reflecting surfaces, the occurrence of interference patterns in the sound waves may adversely affect the clarity of the sound reaching the listener. This effect is the result of the fact that in an automobile interior there is a relatively very short distance between the point of origin of the sound wave (the transducer) and the nearest reflecting surface. The deleterious effect of such interference is more pronounced for middle and higher frequency sounds because the small volume of the automobile interior is more conducive to the creation of interference patterns in the mid- to high-frequency sound waves due to their relatively short wavelengths, which range from a few feet to less than an inch in length. Such interference patterns are much less likely to occur in the low frequency sounds because their wavelengths are roughly one to eight times the length of the typical automobile interior. Accordingly, because the presence of a disproportionate amount of reflected sound makes it more likely that such interference will occur, the benefit of increased sound clarity also results from the placement of the middle- and higher frequency transducers so as to maximize the directly radiated sound reaching the ear of the listener.
One attempt to overcome the shortcomings of the automobile interior as a listening environment has been simply to increase the number and location of full-range loudspeakers so as to insure that the ratio of direct to reflected midto high-frequency sound reaching the listener's ears may be greatly increased. The drawback of this solution is that the additional loudspeakers placed in additional locations will prevent the listener from perceiving two discreet audio signals, thereby defeating rather than enhancing the stereo image due to the lack of separation of the stereo channels.
Another attempted solution has been to locate the entire loudspeaker system as close to the listener's ear as possible. While such a solution might be workable when highly compact loudspeaker systems consisting of a single full-range transducer are acceptable, it is not a practicable one where high-fidelity, multiple-transducer loudspeaker systems are employed, for such systems are of such such a size as to prohibit their placement in those locations which would optimize sound clarity and stereo imaging, e.g., in the upper portions of the front doors or in the dashboard.